PHYSICS 7408/1 Paper 1

Section A

Answer all questions in this section.

1. The neutral lambda particle Λ0 is a baryon with a strangeness of −1.
   One possible decay for a Λ⁰ is

1.1 Deduce the quark structure of a Λ0

Answer:

The Λ⁰ particle is a baryon, which means it is composed of three quarks.

• Strangeness: Since the Λ⁰ has a strangeness of −1, it must contain one strange quark (s).
• Charge: The strange quark has a charge of −1/3. To ensure the overall charge is zero, the
  remaining two quarks must balance this charge. The only combination of quarks that
  achieves this is an up quark (u) with a charge of +2/3 and a down quark (d) with a charge
  of −1/3. Therefore, the quark structure of the Λ⁰ particle is:

1.2 State and explain which interaction is involved in this decay.

Answer:

The decay is mediated by the weak interaction, which allows for the change in quark
flavor and the associated change in strangeness.
Strangeness is changing from -1 to 0 because strangeness is not conserved in weak
interaction.

1.3 An antiparticle of the neutral lambda particle decays into a neutral pion and particle X.
Identify X.

Answer:

To identify X, we can use the conservation of charge and baryon number:
Charge: Λ0has a charge of 0, π0 has a charge of 0, so X must have a charge of 0.
Baryon number: Λ0 has a baryon number of -1, π0 has a baryon number of 0, so X must have
a baryon number of -1.
The particle that fits this description is the antineutron (n-).